The invention relates to a pyrometallurgical process for the reprocessing of irradiated nuclear power reactor fuel elements. More specifically, the invention relates to a pyrometallurgical process for the separation of the transuranic elements from fission product elements. Still more specifically, the invention relates to an improved pyrometallurgical process for the separation of the transuranic elements, neptunium, plutonium, americium and curium, from certain fission product elements, the lanthanide elements and yttrium, contained in a fused salt waste solution.
The disposal of radioactive waste which results from the reprocessing of irradiated nuclear power reactor fuel elements is one of the major problems facing the nuclear power industry today. One approach is to solidify the radioactive waste as it comes from the reprocessing facility into a stable solid material which can be stored in the earth for a period of time sufficient for the radiation to decay to acceptable levels. However, storage times required for spent reactor fuels to achieve such levels of radioactivity are on the order of one million years. This is far longer than the geologic stability of a waste repository can be expected to be maintained. One solution is to remove the extremely long-lived or very hazardous radioactive components, such as the transuranic elements neptunium, plutonium, americium and curium from the wastes so that the remaining radioactive elements, representing the bulk of the radioactive waste, need only be stored for up to 1,000 years before the radio-activity decays to radioactive levels of the uranium used in making the original fuel. It is reasonable to ensure the integrity of a repository for 1,000 years. The actinides thus recovered from the waste can then be reprocessed and recycled to provide additional fuel for nuclear reactors and for isotopic power sources.
A solution to the problem of the disposal of highly radioactive nuclear waste is suggested in an article entitled "Rekindled Interest in Pyrometallurgical Processing", Chemical Engineering Progress, p. 35 (Feb. 1986). Described therein is a reactor concept called the Integral Fast Reactor (IFR). The IFR is a complete, self-contained, sodium-cooled, pool-type fast reactor fueled with a metallic alloy of uranium, plutonium and zirconium, and is equipped with a close-coupled fuel cycle.
Close-coupling of the reactor and the fuel cycle facilities is achieved by locating the reactor and the reprocessing, fuel refabrication, and management of fission product wastes on one site. With this arrangement, it is not necessary to ship fuel to or from the reactor site. As conceived, fission product wastes would be processed and stored on site for long periods of time, perhaps the life of the reactor, before shipment to a waste repository for ultimate disposal.
A pyrometallurgical process utilizing electrorefining for purification of the core fuel has been developed to reprocess the reactor fuel. In this process, the chopped fuel rods are dissolved, or transferred by anodic dissolution, to molten cadmium contained in the low-carbon steel container of the electrorefining cell. The container and cadmium become the positive electrode (anode) of a electrolytic cell. Above the cadmium is a fused molten salt electrolyte made up of chloride salts having high chemical stabilities, e.g. LiCl, KCl, NaCl, BaCl.sub.2 and CaCl.sub.2. The negative electrode (cathode) is a metal rod or a pool of liquid cadmium in a nonconducting container that extends from the top of the electrorefining cell into the electrolyte to within a short distance from the surface of the cadmium. Small amounts of uranium and plutonium are placed into the electrolyte by oxidizing them chemically from the cadmium solution.
Application of an appropriate voltage across the electrodes transfers uranium and plutonium from solution in the cadmium to the cathode, leaving noble metals behind in the anode. Rare earth, alkaline earth, and alkali metal fission products remain in the salt as do a small quantity of the transuranic elements. The cathode deposits are subsequently removed from the electrorefining cell and melted to effect separation from adhering electrolytic salt. After final adjustments of the alloy composition are made, the alloy product is cast into fuel pins, which become fresh fuel for the IFR.
Disposal of the electrolyte remains a problem because it contains small amounts of long-lived transuranic elements, in addition to the shorter-lived fission product elements.
The current proposed process for treating the waste IFR salt does not recover the contained actinides, but converts the wastes into more readily disposable forms. The waste salt is contacted with a cadmium-lithium alloy, a strong reductant, to transfer nearly all of the actinides from the salt into the metal phase. This also results in most of the rare earth fission products being transferred into the metal phase. The treated salt is dispersed in a cement matrix that is cast into corrosion-resistant metal containers. This waste is highly radioactive because it contains fission product cesium and strontium, but it may not require disposal in a deep geologic repository because it does not contain significant amounts of transuranic elements. The cadmium-lithium alloy that contains the actinides and rare earths extracted from the salt is combined with other metal wastes. The mixture is retorted to vaporize the cadmium and leave a metallic residue consisting of fission products, small amounts of actinides, zirconium from the fuel alloy and fuel cladding hulls. This residue is combined with a metal powder, such as copper, and pressed into a solid ingot. The metal matrix is encapsulated in a corrosion resistant container and, because it contains small, but significant amounts of TRU elements, it must be buried in a geologic repository.
One of the long term goals of the IFR is to produce only non-TRU wastes. However, clean separations of TRU elements, especially americium and curium, from the rare earths are difficult to achieve by any known chemical or pyrochemical technique. Therefore, what is needed, is a process compatable with the above described electrochemical process, which will provide a nearly quantitative separation of the transuranic (TRU) elements from the fused electrolyte salt, so that the amount of TRU-contaminated waste which must be disposed of can be greatly reduced or eliminated altogether.